[He can sense Spike the same way he can sense any dead thing - whether they're walking upright or not. So he's not entirely surprised when the vampire comes into view. He even gets ready to say hello.
Except something pulls him up short. He can sense souls too. Or, in this case, when they're missing. He lowers his hand mid-wave and just awkwardly, stands there. Should he tell someone, get help, say something?]